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Pinion pitting question ---continued--attn :carburize

Pinion pitting question ---continued--attn :carburize

Pinion pitting question ---continued--attn :carburize

(OP)
From experts who deal with gear failures, I heard that the pitting usually starts from the dedendum due to negative sliding conditions.But in pitch line its more of a rolling contact that that of a sliding contact.Will you brief me why you have said that the pitting starts on pitch line for me to understand the subject better.If possible you can send the info to r.karthik@flsmidth.com or karts_r@rediffmail.com

RE: Pinion pitting question ---continued--attn :carburize

Rkarthik,
Just in response to the experts.  I think and this
is only a guess is that the material actually is
cold worked in the approach action and at the pitchline
the material is squeezed back and then goes into
the recess action mode.  I once suggested that a trial
modification be made by relieving the pitch line and
see if the phenomenom disappears.  I have seen all
kinds of other reliefs but never at the pitch line.
Seem possible?  As you are aware the approach action
is a push action and the recess action if a pulling
action. For you earlier question, I would try
using one teeth less in the pinion and modify the
addendum by 50 percent to run at the same center
and see what improvement is made in the running
of the parts.  I have not seen any calculations
allowing greater life because of this, but
we have all seen great improvement in using long
addendum pinions where the pinions are the drivers.
Wish I had a lot of money to experiment with this
phenomenom.  I have had great customer success when
they have changed over to recess action gearing.  There
is a large body of research verifying this but nothing
quatitative as far as increasing the life.  But I have
not been spending too much time at the library checking
out all of the current research.  Geat Question!

RE: Pinion pitting question ---continued--attn :carburize

There are many damage mechanisms which affect gear teeth. As the teeth start to mesh together the relative motion between the two surfaces causes sliding then the teeth roll along the pitch line, then they slide back out of mesh.
The challenge is to separate the surfaces by lubrication during the sliding action. If the surfaces are not separated, metal to metal contact occurs and galling and general wear tend to take place. This damage is different from the fatigue pit growth on the pitch line.

RE: Pinion pitting question ---continued--attn :carburize

Rkarthik,

Pitting starts where the combined stresses are highest. In contact stress, this includes stress from contact pressure and traction forces. Rember that oil, under the high pressure of Hertizan contact, has the viscosity of glass. Even if the surfaces are seperated by an oil film, there are traction forces proportional to sliding.

Curvatures are highest at the root, least at the OD. There are 4 variables at work, but in the end it's stress, and this has a dip at the operating PD, due to 0 sliding there.

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